In recent years, active matrix operation systems using, for example, a thin-layer transistor, have been actively developed by virtue of their excellent display performance. Among them, a so-called TFT (thin film transister) display system is the most representative system, and further progress of its display performance have been made. Together with such progress, it is required to improve various properties at the same time with respect to liquid crystal alignment films. As liquid crystal alignment film properties usually required for a so-called TFT display system, most basic necessary properties are, for example, the tilt angle of liquid crystal molecules, voltage holding ratio and charge-accumulation property by a direct current voltage. Alignment film materials which satisfy all of these properties have been desired. Namely, it is required to develop a treating agent for liquid crystal alignment by which a liquid crystal alignment film which is high in the tilt angle and voltage holding ratio and brings about sufficiently low charge-accumulation by a direct current voltage, is obtained.
In conventional treating agents for liquid crystal alignment, film properties are generally improved by employing one type of a polyimide or a polyimide precursor and selecting its structure from various ones.
It is known that all these properties are affected by the structure of the polyimide to be used, and it is the actual circumstance that all the properties of the alignment films are not adequately satisfied by only one polymer structure. Namely, with respect to the conventional alignment treating agents, for example, one agent can exhibit excellent properties of the voltage holding ratio, but is low in the tilt angle or large in the charge-accumulation property, and another one is excellent in the tilt angle and the voltage holding ratio, but large in the charge-accumulation property. Accordingly, no material has been found by which these free properties are satisfied adequately and at the same time.
Further, for the practical mass production of liquid crystal devices, not only the above basic properties but also various properties such as adhesive properties to a substrate, printing properties and rubbing resistance are required.
Namely, it is required to develop various properties including productivity as well as the above basic properties, and it is the actual circumstance that adequate performance can not necessarily be obtained only by a single polyimide resin.
Conventional treating agents for liquid crystal alignment are generally, for example, the one obtained by coating a varnish having a solvent-soluble polyimide dissolved in a solvent on a substrate, and baking it to form a polyimide film, and then rubbing treated it to form a liquid crystal alignment film, or the one obtained by coating a polyimide precursor solution on a substrate, and baking it generally at a temperature of at least 150.degree. C. for imide-modification to form a polyimide film, and then rubbing treated to prepare a liquid crystal alignment film.
Here, it is known that liquid crystal alignment films formed by a solvent-soluble polyimide are generally excellent in the voltage holding ratio, but such films have problems that the tilt angle is not adequate or its stability is not adequate, or the charge-accumulation property is not adequately low. Further, such films are known to be poor in the adhesive properties to a substrate and the printing properties.
On the other hand, the ones employing the polyimide precursor have advantages that a high and more stable tilt angle can be obtained and the charge-accumulation property can be made small, or advantages that the adhesive properties to a substrate and the printing properties are excellent. However, when the voltage holding ratio is poor and further the imide-modification degree is not sufficient, there are disadvantages that, for example, the solvent resistance of the film surface is poor.
Namely, both the solvent-soluble polyimide and the polyimide precursor have advantages and disadvantages contrary to each other as a liquid crystal alignment film. Accordingly, it is not necessarily easy to satisfy all the properties required for alignment films for a so-called TFT.
As mentioned above, excellent treating agents for liquid crystal alignment which satisfy the properties such as the voltage holding ratio, tilt angle and charge-accumulation property at the same time and adequately, have been desired. Namely, it is an object of the present invention to provide a treating agent for liquid crystal alignment which is excellent in the film properties such as voltage holding ratio, tilt angle and charge-accumulation property, or excellent in the adhesive properties to a substrate and the printing properties.